A variety of methods have been disclosed as purification methods for process waste gases, wherein mainly thermal purification methods have been used in the prior art.
For example, a method has been disclosed wherein process waste gases are oxidized or thermally decomposed in a reaction chamber. This takes place by means of a flame fed by a combustible gas and oxygen. The waste gases leaving the reaction chamber pass into a washing or sorbtion chamber in which the solid and/or soluble constituents are washed out of the waste gas with a sorbtion agent. Hydrogen or alternatively natural gas, for example, may be considered as possible combustible gases. The waste gases, now purified of harmful or poisonous constituents, are then drawn out into the atmosphere by an exhaust system.
Process waste gases, in particular waste gases enriched with harmful substances from chemical vapor deposition (CVD) and/or etching processes and chamber-cleaning processes, are purified by such a known method so that pollution of the environment with harmful or possibly toxic substances is avoided. In particular, process waste gases from systems for chemical processing of semiconductor substrates for the production of microelectronic components by means of low-pressure CVD processes (LP-CVD) may be purified or converted into harmless substances by such a method.
Such a waste gas purification method follows, for example, from EP 0,347,753 B1, according to which process waste gases are burned in a reaction chamber under excess oxygen and are passed through a washing or sorbtion chamber to an exhaust system. DE 195 01 914 C1 describes a similar process waste gas purification arrangement. In this arrangement, an outer cylinder encloses a combustion chamber. The outer cylinder also contains a burner with a burner flame, which is directed downward. A sorbtion chamber lies on top of the combustion chamber. The waste gases leaving the combustion chamber are conducted upward within the outer cylinder into the sorbtion chamber and are carried away through a filter by an exhaust system into the atmosphere. For intensive wetting of the waste gases flowing through the sorbtion chamber and reliably washing the solid reaction products from the gases, the sorbtion agent, e.g. water, is sprayed against the direction of flow of the waste gas. The sorbtion agent may, for example, alternatively be sprayed conically against the direction of flow of the waste gas. Then, the solid reaction products are washed downward along the inner wall of the outer cylinder and passed into a separating system for the sorbtion agent.
The process waste gases coming from a low-pressure CVD plant may, for example, contain SiH4, PH3, and N2O in variable oxidation states and concentrations, as well as oil vapors and particulate matter (such as SiO2). These process waste gases are burned in the flame of an oxyhydrogen burner, which is operated with excess oxygen. Instead of the oxyhydrogen burner, which preferably is operated as an internal-mixing burner with a hydrogen/oxygen mixture, a burner operated with natural gas or liquid gas may alternatively be used.
German patent DE-A-19600873 discloses a method for purifying process waste gases by introducing them into a waste gas purification system having a reaction chamber and by post-treating the reaction products leaving the reaction chamber in a washing or sorbtion chamber with an associated washing agent circuit. This reference also shows regulation of the waste gas purification method (Col. 8, lines 20–28). However, this reference uses only the following information to measure and control the process: pressure, temperature, and volumetric flow of the waste gas supplied, in addition to parameters of wash liquid, flame, oxygen supply, and combustible gas supply.
In view of the above, a need exists for a method of purifying waste gases in which the type and amount of harmful substances in the process waste gas are continuously measured before the waste gases enter the waste gas purification system. In addition, a need exists for a method in which the type and amount of harmful substances in the reaction products that leave the waste gas purification system can be determined directly at the exit of the system, and in which the measuring signals can be used directly in order to regulate the operating parameters of the waste gas purification system.